Alloy



Patented Sept. 8, 1936 UNITED STATES ALLOY Philip T. Stroup, New Kensington,

to Aluminum Company of America,

burgh, Pa., a corporat Pa., assignor Pittsion of Pennsylvania No Drawing. Application February 2, 1935,

Serial No. 4,695

3 Claims.

This invention relates to a method of inhibiting oxidation in copper-free aluminum-base alloys when they are exposed to elevated temperatures designed to improve their physical properties.

During the fabrication of aluminum-base alloys to desired form and in the final thermal treatment, the metal is subjected to temperatures considerably above room temperature for the purpose of altering its physical properties. The period of exposure to the elevated temperatures varies with the size of the article treated and the number of articles charged to the furnace in a particular load. Where the article is of relatively thick cross section a longer time is required to thoroughly heat it than where an article of thin section is treated. Again, where a large load is charged to a furnace, the metal on the exterior of the pile of articles will become heated sooner, and be subjected to the elevated temperatures for a longer period of time than the metal at the center of the charge. When so exposed to a high temperature for a long period of time in the presence of the ordinary atmosphere, certain alloys react with the heated atmosphere with the resultant formation of non-metallic substances. The attack is particularly insidious where it penetrates between the grains of metal thus reducing the efiective cross section of sound metal and weakening the article. While the exact mechanism of the reaction is not known, nor has the composition of the product been definitely established, both process and product appear to be the result of oxidation and they will therefore be herein referred to as such for the sake of convenience in terminology.

One of the objects of my invention is to inhibit the tendency of copper-free aluminum-base alloys to oxidize under the for'egoing described conditions. A particular object is to reduce the oxidation of aluminium-base alloys containing magnesium, chromium and silicon when the same are thermally treated. Another object is to effect this improvement without resorting to the use of special equipment or heat treating practices.

I have discovered that these objects may be achieved through the addition of a very small amount of lithium to the copper-free aluminumbase alloys. For example, I have found that the addition of about 0.1 per cent lithium to an aluminum base alloy containing about 1.25 per cent. magnesium, 0.25 per cent chromium and 0.7 per cent silicon reduced oxidation of the alloy when heated for 16 hours at 960 F. as shown by an elongation which is 160 per cent of that of the same alloy without lithium. The elongation is a reliable test of the progress of oxidation since the formation of oxidation product between the grains embrittles the alloy with a consequent reduction in the elongation.

The members of the chemical group of alkali metals other than lithium, especially sodium and potassium, have been employed in the production of aluminum-base alloys. These metals have been especially useful in altering the structure of aluminum-silicon alloys. The addition of these same alkali metals to the herein described alloys does not, however, diminish oxidation at elevated temperatures. Lithium as a member of the alkali metal group behaves in an altogether unique manner in this respect, it having a beneficial effect whereas the other elements of the group have either ,a neutral or adverse influence upon the tendency to oxidize athigh temperatures.

For the purposes of my invention the presence of from about 0.005 to 0.15 per cent lithium in the alloy is sufficient to inhibit oxidation. Under particularly severe conditions as much as 0.25 per cent of this element may be used. The lithium may be added either in metallic form or as the carbonate; other methods of incorporating the element in the alloy have not proven to be satisfactory. The addition of this element is preferably made shortly before the alloy is poured rather than at an early stage in the melting procedure.

The addition of lithium to the alloy afiords protection against oxidation where the alloy is heated between about 600 and 1050 F. for periods up to about 20 hours. The beneficial effect of lithium diminishes beyond these limits of temperature and time.

Aluminum-base alloys consisting of aluminum, about 0.5 to 4 per cent magnesium, 0.1 to 1 per cent chromium, and 0.5 to 4- per cent silicon are subject to oxidation on continued exposure to elevated temperatures, and I have found that the addition of lithium to this type of alloy greatly reduces the tendency to oxidize. The presence of magnesium appears to induce the oxidation since alloys without this element seem to suffer less when heated for any length of time. As an example of the benefit to be derived from the use of lithium, the case of heating an aluminum-magnesum-silicon alloy may be cited. The normal alloy contained about 1.25 per cent magnesium, 0.25 per cent chromium, and 0.7 per cent silicon. When heated at 960 F. for 4 hour and quenched, the alloy had a tensile strength of 32,200 pounds per square inch, a yield strength 56 of 16,900 pounds per square inch, and an elon- The term aluminum as herein employed refers gation of 22.5 per cent in 2 inches. When heated to the commercially available metal which conquenched, the alloy lost about 16 per cent in though the silicon later becomes an intentional tensile strength, 19 per cent in yield strength and alloying constituent in the flnal product. 5

63 per cent in elongation. The same alloy con I claim: taming 0.1 per cent of added 11 um under the 1. An aluminum-base alloy consisting of alusame heat treatment had an initial tensile minum, about 0.5 to 4 per cent magnesium, 0.1

strength of 18,300 pounds per square inch, and and 0.005 to 0.25 per cent lithium, said alloy being 10 an elongation of 24.3 per cent. After heating at characterized by resistance to oxidation between 960 F. for 16 hours, the tensile strength dropped about 600 and 1050 F. when exposed to said about 12 per cent, the yield strength about 7 p a u s f pe D to about 20 hoursper cent and the elongation only 45 per cent. 2'. An aluminum-base alloy consisting of alu- The foregoing data clearlyreveal the bene-. minum, from about 0.5 to per cent magnesium, l5

effect of lithium becomes more pronounced as 1 per cent silicon, 0.1 per cent lithium and the the length of the exposure increases. Lithium balance aluminum, said alloy being characterized does not provide protection to the alloy for an by resistance to oxidation when heated to be- 25 periods of time employed in thermally treating PHILH? T. STROUP. aluminum-base alloys. 

